1. Field of the Invention
The present invention relates generally to production of training signals with cyclic extension used during receiver/transmitter initialization.
2. Background Art
Very high speed digital subscriber loop (VDSL) systems provide the next generation of high speed networking technology over existing telephone line infrastructure. VDSL resolves many of the technical challenges confronting its predecessor systems, such as digital subscriber line (DSL), high bit-rate digital subscriber-line (HDSL), and asymmetric digital subscriber-line (ADSL) systems. For example, VDSL based systems can provide very high symmetric and asymmetric bandwidth, up to 65 mega bits-per-second (Mbps), on a single telephone wire pair. Bandwidths of this order are capable of accommodating many bandwidth intensive services, such as video on demand, high definition television (HDTV), and broadcast video, now being deployed by telecom service providers.
Discrete multi-tone (DMT), a multi-carrier modulation scheme, is one of the modulation techniques selected for use within VDSL systems. DMT is preferred by service providers, over other available modulation schemes, because of its performance, flexibility, and compatibility with existing infrastructure and systems. Regardless of the modulation scheme, however, several technical challenges confront transmitting these bandwidth intensive services over primarily copper lines.
Inter-symbol interference (ISI), caused by the overlap of data symbols in time at the receiver, and frequency dependent noise, in the form of echo, are two of the challenges that confront transmitting high bandwidth services over existing VDSL communications channels.
To avoid ISI, for example, cyclic extensions (e.g. cyclic prefix and cyclic suffix) are added to every DMT symbol prior to transmission. The occurrence of cyclic extensions, particularly during an initialization (i.e. startup) stage of DMT communications channels, is important within the context of DMT receivers and transmitters. Of particular concern within this context is the applicability of signals with cyclic extension during the initialization stage when only the first upstream band (US0) is available.
During the initialization stage, central office (CO) and customer transceivers make measurements of received signal amplitude and/or power which are used to set the automatic gain controls and to signal power cutback levels to the other transmitter. Measurements and calibrations during this stage will effect the operability of a communications channel during the remainder of a communications session.
Several problems have been identified that occur within conventional DMT systems during this initialization stage. By way of background, a source of one such problem is the signal structure requirement that is identified, for example, in the G.992.3 standard definition of a C/R-COMB signal. Here, three identical symbols are used, but without cyclic prefix, without cyclic suffix, and without windowing. This approach is inadequate for VDSL.
Another source is the STMicro ITU-T Contribution D-1015 Geneva April 2004. This source advocates production of a periodic signal only in a small part of the band, but does not modulate any data in this band. Therefore, on a long copper loop, for example, it is difficult to transmit data when the specifically required frequencies in this small part of the band might not be available. Since this approach cannot accommodate the modulation of data, it is not a suitable approach for eliminating challenges, such as ISI and echo, mentioned above.
What is needed, therefore, is a method and system to ameliorate the challenges of the conventional telecommunications systems noted above. More particularly, what is needed is a method and system for effectively increasing the cyclic prefix and cyclic suffix while using for example, implementations of fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT).